A common type of hydraulic differential comprises a first axial piston hydraulic machine of variable displacement coupled to a second axial piston hydraulic machine of fixed displacement in such a fashion that driving one of the hydraulic machines at a certain rotational speed with a source of mechanical power whilst changing the phase and amplitude of displacement of the variable displacement machine relative to its porting causes a change in rotational speed of the undriven hydraulic machine. This operation is particularly useful for converting a source of mechanical power that has a variable rotational speed to a constant rotational speed. By changing the displacement of the variable hydraulic machine in proportion to the mechanical source speed changes, the output of the undriven hydraulic machine may remain constant.
It is common to configure the mechanical and hydraulic coupling of the hydraulic machines or units in such a hydraulic differential so that the variable and fixed displacement units have common rotational axes. Such an end-to-end arrangement is very compact and permits direct hydraulic coupling between the blocks of the hydraulic units with suitable port and valve plates. However, such an end-to-end arrangement also tends to generate high hydraulic thrust forces between the interfaces of the hydraulic units since they generate significant working pressure in operation. Consequently, the hydraulic differential requires large bearings to withstand these hydraulic thrust forces, adding to cost, weight and size of the hydraulic differential.
One design of hydraulic differential has a configuration that contains these hydraulic thrust forces in such a way that the differential does not require large bearings is described in Iseman, U.S. Pat. No. 4,794,756. This differential has a structure that clamps the wobbler for the fixed displacement hydraulic unit to the block and piston set for the variable displacement hydraulic unit, thereby containing all the hydraulic thrust forces so that it does not require large bearings. One disadvantage is that the variable and fixed port plates needs to be shimmed close together to minimize leakage between the variable and fixed blocks port plates or the piece parts that clamp around the port plate need to be machined to impractical tolerances.